Monitors the receiver's battery voltage and acts as a kill switch when the voltage is low (or reaches zero).
We all pretty much know about Fail Safe devices, but many of us don't know that they have limitations. Even Fail Safes built into most of today's radios are designed only to return the throttle position to a determined point to shut your engine down.
This works well as long as you have sufficient power from the receiver battery pack to the receiver. A few years back Tony ran into a situation while running one of his customers boats and lost all control of the boat, yet the radio was failsafe equipped.
The boat vilolently ran into a dock and exploded. Needless to say Tony replaced the customers boat. When the boat got on the bench the guys at the shop quickly noticed that one of the battery wires was broken loose from the battery pack yet the battery pack was still secure in the radio box holder.
After soldering the wire back into place the radio system began to work as so did the fail safe when tested. Tony then went to the drawing board to develop a safety device unlike any other at the time. What was not being addressed was the fact that you have to have voltage/power present to the receiver to enable it to return to the off or throttle-down position. As the power depleted the fail safe did it's job shutting the throttle down, but in the case where the power was at a total loss the boat had no alternative but to respond to the last comand received.
The EKG solves the problem by not getting directly involved with the radio system signal in any way. It's only function is to monitor the receiver battery voltage at a threshold for safe operation, or detect it's loss completely. Rather than getting involved with the radio systems function, such as instructing the throttle servo to shut-down, the EKG simply connects between the switch and receiver, with two additional wires that connect to the "kill" wire from the engine and the other, ground or the engine block.
If the EKG determines a low voltage or no voltage present it "kills" stalls your engine. Combining the EKG along with the radio's fail safe is probably the best combination to assure the highest standard of protection.